The present invention pertains to an image recording apparatus for recording an image onto a reversible heat-sensitive recording medium having a liquid crystal compound that exhibits a cholesteric phase.
In light of the trend toward reduced energy consumption and environmental protection, rewritable sheets that allow overwriting of information have been developed in recent years for the purpose of paperless or reduced-paper image recording. In addition, various types of rewritable image recording materials have been developed in conjunction with the development of IC cards and prepaid cards. However, the currently available rewritable recording materials are capable only of black and white or single-color display, and a material offering full-color display capability is desired.
One rewritable compound that has a full-color display capability is known and is based on a high-molecular weight cholesteric liquid crystal compound. However, this material has not been commercialized because it has a slow response time during recording. Additionally, a second compound has been proposed that has an improved response time during recording and which is based on a low-molecular weight cholesteric liquid crystal compound. This liquid crystal compound exhibits a cholesteric phase in regions that are warmer than room temperature, and these temperature regions display a reflection color corresponding to the temperature. When these regions are cooled rapidly, the reflection color present just prior to the rapid cooling is preserved and fixed. In other words, by heating a sheet that is uniformly coated with this compound at various different temperatures and then rapidly cooling it, the display color may be freely selected and multi-color or full-color information (images) may be obtained. Moreover, by re-heating the sheet to a certain uniform temperature and cooling it, the images may be erased.
In regard to the image bearing medium, Leuco dyes with a developer and a subtractive agent, organic low molecular liquid crystal dispersed in high-molecular resin and high-molecular cholesteric liquid crystal are known as conventional rewritable thermosensible recording materials.
A Leuco dye with a developer and a subtractive agent develops a color as the lactone ring contained in the Leuco dye molecules opens, and loses the color as the lactone ring closes. The lactone ring opens when the material is rapidly cooled after being raised in temperature, and closes when the material is slowly cooled. Such a Leuco dye is coated on a sheet member, and information is written thereon with a thermal head and erased therefrom with heat rollers.
A well-known type of organic low molecular liquid crystal dispersed in high molecular resin uses BA (behenic acid) as the organic low molecular compound and uses PVCA (polyvinylchloride polyviynlacetate copolymer) as the high molecular compound. This material can be switched between a transmitting state and a scattering state in accordance with the heating temperature and can maintain the state after being cooled. Information can be written in this material with a thermal head.
High molecular cholesteric liquid crystal polymerized with a vinyl compound having a cholesteric liquid crystal compound as a side chain is known. This material can be caused to change the display color by being heated beyond a crystallization temperature and thereafter being cooled rapidly from a predetermined temperature.
Such a Leuco dye with a developer and a subtractive agent can develop only those colors determined by the Leuco dye and cannot develop full colors for a desired image. The organic low molecular liquid crystal dispersed in high-molecular resin, which displays a color by switching between the transmitting state and the scattering state, cannot develop full colors either. The high molecular cholesteric liquid crystal, in which the developed color can be changed basically in accordance with the heating temperature, requires time on the order of minutes for changing the color, which poses a large stumbling block to practical applications.
Under the circumstances, a rewritable thermosensible recording medium which a full-color image can be written on and erased from at a practicable rate is yet to be successfully developed.
In the meantime, for an overhead projector (OHP) which makes a display by projecting an image, OHP sheets which are transparent plastic films are used. On an OHP sheet, an image is recorded with a writing tool such as an oil pen or with an image forming apparatus such as a copying machine, a printer or the like.
However, such OHP sheets are expensive compared with copy sheets and are difficult to reuse. Thus, conventional OHP sheets have problems in cost and resource saving. Also, because conventional OHP sheets are transparent, the background can be seen through. Therefore, the image on an OHP sheet and the background are seen overlapped, and it may be more difficult to recognize the image on an OHP sheet than to recognize the image on a sheet of paper.
Accordingly, the object of the present invention is to provide an image recording apparatus that can efficiently write high quality images on a recording medium in which the display color changes in response to the heating temperature, and the display color is fixed by rapidly cooling the sheet down from the previous heating temperature.
In order to attain this object, the image recording apparatus pertaining to the present invention comprises a heater which selectively heats a recording medium. For the recording medium, a sheet having a recording layer comprising a low-molecular weight cholesteric liquid crystal compound is used. This liquid crystal compound has the property that its display color changes in response to the temperature to which it is heated, and the display color is fixed when it is cooled rapidly from the previous temperature. Therefore, by controlling the heating temperature of the heater, the display color may be freely selected, enabling a black and white, single-color or full-color display to be obtained. Moreover, by heating the recording layer to a certain temperature and then cooling it either rapidly or slowly, a transparent or scattered-molecule state may be selectively achieved, erasing the previously recorded information.
When performing full-color image recording, writing must be performed by changing the heating temperature of the heater while it is moved forward and backward. In this case, if only a single heating element is used for all colors, the applied voltage to the heating element must be controlled very precisely. Therefore, in the image recording apparatus pertaining to the present invention, it is preferable for the heater to have multiple heating elements having different heating temperatures located at prescribed distances from one another in the scanning directions. In other words, the multiple heating elements are controlled in a binary fashion so that each may be selectively turned ON or OFF, and where their heating temperatures are set to red, green and blue, respectively.
In a construction as described above, where image writing is performed through the scanning by multiple heating elements having different heating temperatures, if image writing is performed in a sequential fashion with the elements having the highest heating temperature going first, later there are no changes in previously written images due to subsequently written images (heating), and image quality improvement may be attained. To increase recording speed (i.e., reduce recording time), it is preferred that the heating elements be controlled so that the image may be written during both the forward and backward movement of the thermal head. When this is done, however, the heating temperature sequence must be maintained during the forward and backward movement of the thermal head. The heating temperature sequence during the forward and backward movement of the multiple heating elements may be kept constant by controlling the voltage applied to each heating element or by rotating the thermal head 180 degrees.
On the other hand, it is preferred for the image recording apparatus of the present invention to have an erasing means that erases an image which has been previously recorded on the recording medium. This enables the quality of newly-recorded images to be improved. Furthermore, if the image recording apparatus has a preliminary heating means that heats up the recording medium prior to image recording, the recording process can be made more efficient.
Another object of the present invention is to provide an improved rewritable image bearing medium.
Another object of the present invention is to provide an image bearing medium on which full-color display is possible.
Another object of the present invention is to provide an image bearing medium on which only a short time is required to make a full-color image.
Further, another object of the present invention is to provide a rewritable image bearing medium which can be used as a document for a projector such as an OHP sheet.
In order to attain the objects above, an image bearing member according to the present invention comprises a substrate and an image bearing layer disposed on the substrate. This image bearing layer contains a low molecular compound which selectively exhibits a solid phase and a thermosensible cholesteric liquid crystal phase depending on its own temperature, and the low molecular compound changes from the thermosensible cholesteric liquid crystal phase to the solid phase keeping the state set in the thermosensible cholesteric liquid crystal phase when changing its temperature rapidly.
According to the present invention, by raising the temperature of the low molecular compound in accordance with image information, a display of a desired color can be made on the image bearing member at a high speed, and further, a full-color display can be made. Also, by heating the low molecular compound again, the information displayed on the image bearing layer can be erased.
When a transparent material is used as the substrate, the image bearing medium can be used as a document for an image projector such as an OHP sheet.
Further, the image bearing medium according to the present invention may have, between the substrate and the image bearing layer, and intermediate layer which has a smooth surface in contact with the image bearing layer. The intermediate layer may be imparted with a function of absorbing at least a component of light.
Also, the image bearing medium may be a composite of a low compound and a resin material. Moreover, the image bearing layer may contain spacers to maintain the thickness of the image bearing layer.
In this specification, thermosensible cholesteric liquid crystal means a compound which has a characteristic of changing its chiral pitch as changing its temperature within a specified range. Accordingly, as thermosensible cholesteric liquid crystal changes its temperature, the wavelength of light reflected thereby changes. Further, a thermosensible cholesteric liquid crystal phase means a phase wherein thermosensible cholesteric liquid crystal exhibits the above-described characteristic.